The logic for e-books and physical books is incomplete

[thoni56]

A book might be available as

• a printed book (even multiple copies might be available, actually)
• an e-book (possibly in multiple formats)
• both

When a book is only available as an e-book, it should not indicate any physical presence ('[i biblioteket]'). Nor should it be possible to borrow it.

To solve this we probably need some metadata indicating if it is available as a (or multiple) printed books.

[thoni56]

Just added conditions so that a book is assumed to be an e-book if it has a file which means it cannot be borrowed or have a borrower. Else it is assumed to initially be in the library and can be borrowed.